1. Field of the Invention
The invention relates to a technology by which a plurality of communication terminal devices communicate with one another.
2. Description of the Related Art
With the miniaturization and weight saving of information terminals in recent years, it has become common to carry information devices around. Considerable research has thus been done on the construction of a wireless ad-hoc network as what is called on-demand communication.
Since the ad-hoc network requires no base station or access point, it is easily possible to construct one even in locations with no such infrastructure.
For example, by using this ad-hoc network, a plurality of users can bring their portable game consoles to hold mutual wireless communication and enjoy a game together.
An ad-hoc network is constructed by terminals communicating with one another through the use of IEEE 802.11, Bluetooth, and other technologies. Unless external power supply is available full-time, portable terminals are driven by limited amounts of battery power. It is thus preferable to suppress battery consumption as much as possible. For that purpose, power control processing in power saving mode is standardized even in such communication standards as IEEE 802.11.
FIGS. 1A to 1D are timing charts showing station operations in a power saving mode, standardized in IEEE 802.11.
As shown in FIGS. 1A to 1D, one of stations (wireless communication terminal devices) STA, STB, STC and STD (STA to STD) initially transmits a beacon signal BCN. The beacon signal BCN is an annunciation signal which is communicated to all the stations.
A time window called ATIM (Announcement Traffic Indication Message) window is started in succession to the transmission of the beacon signal BCN. This window shows the time during which the nodes must be kept active.
In the power saving mode of the IEEE 802.11 standard, each of the stations can transmit an ATIM signal during the ATIM window so as to prevent other station(s) from sleeping.
In the example of FIGS. 1A to 1D, the station STB unicasts the ATIM signal to the station STC. The station STC returns an ACK (ACKnowledge) signal for acknowledging receipt to the station STB.
Since the stations STA and STD are not involved in transmission nor reception of the ATIM signal, they can enter a sleep state when the ATIM window ends.
On the other hand, neither of the stations STB and STC can enter sleep. After the end of the ATIM window, the station STB transmits data to the station STC. Receiving the data, the station STC returns the ACK signal to the station STB.
Before the end of this beacon interval BCNI, the stations STA and STD are activated to transmit or receive another beacon signal BCN. In the next ATIM window, none of the stations transmits or receives any ATIM signal. After the end of the ATIM window, all the stations STA to STD are thus in the sleep state.
The timing charts of FIGS. 1A to 1D have dealt with a quite simple case, for the sake of explaining the power saving mode of the IEEE 802.11 standard. When a plurality of portable game consoles construct a network, however, status information on each individual game console must be exchanged mutually, and thus more signals are communicated. In game applications that demand highly real-time responses, the status information must be updated frequently. It is thus preferable to transmit data via multicast communication.
As described previously, in unicast communication, whether or not data is actually transmitted properly is determined depending on if the ACK signal arrives from the reception side. When the ACK signal does not arrive, the data can be retransmitted on the assumption that there is a communication failure. In multicast communication, on the other hand, the absence of the ACK signal makes it impossible to check if data is delivered to the destinations. For that reason, multicast communication employs the method of keeping transmitting the same data for surer data transfer.
Nevertheless, the use of the foregoing method has the disadvantage that there is no way to check if transmitted data is received by the other stations (terminals). This means an increase in power consumption since the data is kept transmitted constantly even if it is received by the other stations (terminals). This increase in power consumption has an impact not only on the transmission side but also on the reception sides.
For example, as shown in FIGS. 2A to 2D, despite the successful reception by the other stations (terminals) in interval 1, the transmission is continued even in intervals 2, 3, and 4. Much of the transmission processing is unnecessary, merely increasing the power consumptions.
As seen above, if a plurality of stations (terminals) are involved in wireless communication, exchanging data by using multicast packets has the following two disadvantages.
A first disadvantage is an increase in power consumption ascribable to the continuous transmission. This increase in power consumption has an impact not only on the transmission side but on the reception sides as well.
A second disadvantage is that there is no means to know if data is received by the destinations due to the data exchange in multicast packets.